Direct view storage system



Sept. 30, 1969 w HELT ET AL 3,470,414

DIRECT VIEW STORAGE SYSTEM Filed May 27, 1968 F- NEGATIVE E FLOOD GUN A CUTOFF WITNESSES I INVENTORS Wi son G. Helt and W Jock K. smnh BY g ATTORNEY United States Patent ABSTRACT OF THE DISCLOSURE This invention relates to storage tubes and particularly to improved circuit and method for erasing a direct view storage tube. More particularly, the invention relates to means for modulating the spot size of the erase electron beam during the erase cycle for achieving complete and uniform erasure during the erase cycle.

BACKGROUND OF THE INVENTION In a direct view storage tube, the tube normally comprises an electron gun for writing information, a flood electron gun for reading and erasing information, foraminous storage grid and a phosphor display screen. The write gun normally writes a positive charge image corresponding to the information onto the storage grid. The flood gun, which provides a beam which floods the en tire storage grid, is operated at a suitable potential during the read operation so that the electrons pass through the foraminous storage grid. The electrons passing through the storage grid excite the phosphor screen to provide a light image corresponding to the information. In this manner a visual display is formed. The display is erased by charging the storage grid sufficiently negative to prevent the passage of the viewing beam electrons. This is normally accomplished by applying positive pulses to the backplate of the storage grid thereby causing the viewing beam electrons to collect on the storage grid.

Display storage tubes are normally found to be subject to inherent persistence nonuniformity. The factors within the tube which may bring about this nonuniformity include differences in the thickness of the dielectric storage coating on the storage grid. The dielectric coating is normally evaporated onto the conductive mesh of the storage grid. Another factor is the varied landing angles of the flood beam electrons on the storage grid. Another factor is ion bombardment of the storage grid which are generated by the electron beam due to residual gas being present in the envelope. Another factor is the density distribution of the flooding electron beam over the storage grid. The elfect found in the display is that there is a nonuniformity of persistence of the image in the normal operation of the tube. It is found that the persistence of the tube is of a relatively short time on the outer edge region of the screen and increases toward the center. Several complex techniques having been employed in the past tend to overcome this problem such as variable transmission mesh and graded dielectric evaporation on the storage grid. However, this concentric nonuniformity still exists due to the natural lens effects and the landing angles of the electrons. In addition, attempts have been made to improve the systems uniformity by altering erase pulse waveforms to encourage all written levels to decay at a similar rate. This has been found to be unsuccessful. It has been found that these inherent nonuniformities provide a longer persistence at the center of the display screen. It is therefore desirable to have more erase time devoted to this area of the display. As the persistence decreases toward the edge of the screen, the erasing time should also decrease in a similar manner.

Patented Sept. 30, 1969 SUMMARY OF THE INVENTION This invention is generally related to a circuit and method for modulating the spot size of the flooding or reading electron beam in a display tube to compensate for nonuniformity of erase or read over the entire storage surface.

It is accordingly a general object of this invention to modulate the beam spot of the flooding gun in a direct view display tube to compensate for nonuniformity in display.

It is another object to modulate the beam size of the erase beam during the erase cycle in a display storage tube in order to compensate for nonuniformity in erase of the charge image on the storage display grid.

It is still another object to provide a means for selectively modulating the beam spot of the erase gun in a display tube in order to provide a variable persistence within the display image.

BRIEF DESCRIPTION OF THE DRAWING FIGURE 1 is a schematic diagram of a direct view storage tube in accordance with the teachings of this invention; and

FIG. 2 illustrates the operation of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, a direct view storage tube comprising an evacuated envelope 10 is illustrated. The envelope 10 consists of an enlarged tubular portion 14 connected by a tapered portion 16 to a tubular neck portion 18 of smaller diameter than the tubular portion 14. The tubular portion 14 is closed at its other end by a faceplate portion 20 of suitable light transmissive material such as glass. The other end of the neck portion 18 is closed by a suitable base portion 19 containing lead-in members for applying voltages to the electrodes provided within the envelope 10. An electron sensitive coating 22 is provided on the inner surface of the faceplate 20-. The electron sensitive coating 22 is the viewing or display screen and may be of a suitable phosphor material which emits light in response to electron bombardment. A suitable phosphor material is zinc sulphide. The phosphor coating 22 is also provided with an electrically conductive coating 26 of a suitable material such as aluminum. A lead 28 is provided to the exterior of the envelope from the coating 26 and is connected to a battery 27. The potential applied to the electrode or conductive coating 26 may be about 10,000 volts positive with respect to ground.

Disposed adjacent to the phosphor screen 22 is the storage grid 30. The storage grid 30 consists of an electrically conductive mesh 32 with a dielectric coating 34 provided on the side facing away from the phosphor screen 22. The conductive mesh 32 is also connected by means of a lead-in 36 to the exterior of the envelope. The lead 36 is connected to a pulse source 39 which generates a positive pulse of about 15 volts in magnitude and of a duration of about 10 microseconds and with a repetition rate of about 1000 cycles per second. Other values may also be used dependent upon the speed of erase desired in the particular application or situation. A suitable source is a Hewlett-Packard pulse generator 212A as described in their catalog 22-A, 1955. The lead 36 is also connected to the positive terminal of a voltage source such as a battery 41. The battery 41 may be of a potential of about 10 volts with respect to ground. The dielectric coating 34 may be of any suitable material such as magnesium fluoride or silica.

Positioned adjacent to the dielectric surface 34 of the grid 30 is a collector mesh grid 40. The grid 40 is also .3 of an electrically conductive material with a lead 42 connected to the exterior of the envelope. The lead-in 42 is connected to the positive terminal of a suitable potential source such as a battery 43 with the negative terminal connected to ground. The voltage of the source 43 may be about 200 volts.

An electron gun 50 is positioned within the neck portion 18 for generating and directing an electron beam onto the storage grid 30. The electron gun 50 is a Writing type gun which generates an electron beam of small spot size. The writing gun 50 may be of any suitable construction to provide such a beam and consists of at least a cathode 52 and a control grid 54. The cathode may be connected to the negative terminal of a voltage source 45 of about 2000 volts with the positive terminal connected to ground. The control grid 54 is connected to a signal source 47 with a suitable bias as illustrated. Horizontal and vertical deflection plates 56 and 58 are provided for directing or scanning the electron beam from the electron gun 50 over the grid 30. Suitable deflection voltages are applied to the plates 56 and 58 to scan the elemental electron beam from the electron gun 50 over the storage grid 30.

Also positioned within the neck portion 18 of the envelope is a second electron gun structure 60 which may be referred to as the viewing or reading gun and performs a function of reading and erasing information. The gun 60 provides a large area beam so as to substantially flood the entire area of the storage mesh 30. The flood gun 60 includes at least a cathode 62 and a control grid 64. The cathode 62 may be connected to ground potential. A grid 61 is positioned in front of the control grid and is electrically connected to a wall coating 63. The wall coating 63 extends from the neck portion 18 through the flared portion 16 to the portion 14. A collimating electrode 66 is provided in the form of an electrically conductive coating on the inner surface of the envelope portion 14. In addition, a coating 68 may be provided on the inner surface of the tubular wall 14 adjacent to the collector and may be operated at the same potential as the collector electrode. Representative potentials applied to the coating 68, collimating electrode 66, and the electrode 63 are respectively 200 volts, 80 volts and 100 volts. The control grid 64 may be operated at a potential such as volts negative with respect to ground. The electrode is maintained at this potential by means of a suitable potential source 65 which is also the potential applied to the grid 61. A suitable potential is applied to the control grid 64 by means of a variable potential source 67 which is in turn connected to ground. The coating 66 is supplied witha suitable potential by means of a battery 69 with the negative terminal connected to ground. The coating 68 may be connected to the collector grid 40 and operate at the same potential supplied by the source 43.

In the specific embodiment shown, a pulse source 71 is connected to the control grid 64 and may be of any suitable type source to give the waveform B illustrated in FIG. 2. A suitable potential source to provide the differential pulse of suitable amplitude is described in chapter 2 of the book Pulse and Digital Circuits by Millman and Taub and published by McGraw-Hill.

Prior to writing operation, the conductive mesh 32 of a storage grid 30 is maintained at a quiescent voltage of about 5 volts positive with respect to ground by means of the battery 41. The pulse generator 39 provides a positive pulse to the storage grid 30 only during the erase cycle which may be about volts positive and during application of this pulse, the dielectric storage surface of the storage grid is charged to the potential of the cathode 62 of the flood gun 60. The cathode 62 is connected to ground. The potential difference across the dielectric layer 34 at this time is about volts. At the end of the positive erase pulse from the pulse source 39, the backing electrode 32 returns to its initial positive direct current potential of 5 volts. The charge stored on the dielectric surface 34 changes from ground potential to a negative potential approximately equal to 15 volts due capacitance coupling. This voltage is normally adequate to cut off the tube.

During the Writing operation, the electron gun 50 is modulated by a signal fro-m a suitable signal source and generates a small pencil like electron beam which is defiected over the storage grid by means of the deflection plates 56 and 58. The cathode 52 of the writing gun 50 is generally operated at a potential of a negative 1500 to 2500 volts with respect to ground. The signal source 47 modulates the control grid 54 of the writing gun 50 in accordance with the information to be written onto the storage grid 30. The collector grid is operated at a positive potential of about 200 volts. In those areas, where the electrons from the modulated electron write beam land on the storage grid 30, the electrons have suflicient velocity to produce a greater number of secondary electrons than incident primary electrons. Thus, more electrons leave the storage grid 30 than arrive on those elements of the storage grid 30 struck by the write beam and the storage grid assumes a less negative charge. The secondary electrons emitted from the storage grid 30 are attracted and collected by the collector grid 40. Thus, the storage elements may be charged to any potential intermediate between the storage grid cut off voltage, which in the specific embodiment discussed here is a negative 15 volts, and zero potential. In this manner, a storage pattern may be written onto the storage grid 30 by the writing gun in accordance with the modulation applied to the control grid of the writing gun from the signal source 47.

In the viewing operation, the viewing gun provides a low velocity electron screen which continuously floods the entire collector grid 40 and the storage grid 30. A display with exceptional brightness is possible because of the high viewing gun current. The high current can be obtained because the viewing beam is not controlled by the methods ordinarily employed in cathode ray tube guns and consequently is not limited by focusing, deflection and other modulation requirements. The conductive coatings 63, 66 and 68 collimate the paths of the electrons in the stream before they reach the storage grid 30. It is necessary that these low velocity electrons in the viewing beam approach the storage grid 30 in paths perpendicular to the storage grid. The collector grid 40 and associated coating 68 serves to accelerate the electrons in the viewing beam and to repel any positive ions, which are generated within the volume between the electron gun and the electric grid. The potential on any storage element of the storage grid 30 determines the number of the viewing beam electrons passing through the apertures in the storage grid 30 in the immediate vicinity of the element. When the potential of the storage grid 30 is such as to allow passage of the electrons, these electrons are accelerated by the voltage applied to the viewing screen 22 and strike the screen 22 directly opposite of the storage element. As a result, there is produced on the display screen 22 an image corresponding to the storage pattern written onto the storage grid 30 by the electron gun 50. In those areas, where the Writing beam did not modify the charge from the negative 15 volts, the electrons will not pass through the apertures in the storage grid 30 but will be repelled and collected by the collector 40. In those areas where the potential is reduced from the negative 15 volts, electrons from the flood gun 60 will pass through the apertures and be accelerated to the screen 22. The display screen 26 may be operated at a potential of about 10,000 volts supplied by the battery 27.

The viewing time of the storage image depends on the particular application and may be as high as 60 seconds. In this specific application, the erase pulse source 39 provides 1000 pulses per second with a pulse length of 10 microseconds and a pulse amplitude of 15 volts. This gives a retention time of 30 seconds.

In the erase operation a positive pulse of 15 volts is applied to the backplate of the storage grid 30 from the positive pulse source 39. This pulse source output is illustrated in FIGURE 2 by the curve V which is a plot of the amplitude of the pulse as the ordinate and a time as the abscissa. Simultaneously, with the application of the pulse from 39 to the backplate of 32 is the application of a stepped waveform illustrated by curve B in FIG. 2 from the pulse source 71. The step-like waveform is applied to the control grid 64 of the flood gun 60 with an amplitude according to pulse height A. The size of the beam spot from the flood gun 60 onto the storage grid will be that as indicated by A in portion C of FIG. 2. Portion C of FIG. 2 illustrates the storage grid 30 with the circular curves thereon indicating the outside dimensions of the beam spot from the flood gun 60. When the voltage on the control grid 64 is increased to B, the beam spot will correspond to the area defined by the circle B which also includes the circle A. This stepwise increase of the beam spot from the flood gun 60 continues during the erase cycle as defined by curve V of FIG. 2 until the beam spot covers the entire storage grid as indicated by the circle G and corresponding to the pulse height G applied to the control grid 64.

In the above manner the storage grid 30 is erased in a stepwise manner such that the time that the erase beam is directed onto the storage grid 30 decreases in a radial manner from the center portion outwardly to the outer periphery of the storage grid 30. The stored pattern on the storage grid 30 may be erased by applying to the backing electrode 32 a positive pulse or a series of positive pulses having sufficient amplitude and duration to permit driving the storage surface potential beyond the cut off voltage. The amount of the erase depends on the number of flood electrons landing on the storage grid 30 which is dependent on the density of the electron beam and the duration of the erase pulse on the storage grid.

It is obvious that the waveform utilized and shown 1n curve B of FIG. 2 does not necessarily have to be in his step form. In practice, the waveform contained by integrating the steps is much more desirable both from the circuit standpoint and the display of currents. The generating circuit illustrated as the pulse source 71 may be installed in the equipment or suitable circuitry integrated with the envelope and associated shield structure.

It is also apparent that by altering the wave shape illustrated in curve B of FIG. 2, other persistence patterns could be accomplished. For example, areas E, F and G could be made to ofier longer persistence than A through D by using a diiferent waveform. This method is more effective when using low erase pulse duty cycles or operating in a high contrast mode since the light burst during erase is at a minimum brightness level.

Although the present invention has been described with a certain degree of particularity, it should be understood that the present disclosure has been made only by way of example and that numerous changes may be resorted to without departing from the spirit and scope of the present invention.

We claim as our invention:

1. A storage system comprising a storage tube including a storage electrode, a large area flooding electron beam gun including a plurality of electrodes for directing a flooding electron beam onto such storage electrode and means for periodically varying the diameter of said flooding electron beam to selectively vary the number of electrons approaching said storage electrode over the area of said storage electrode.

2. The storage system set forth in claim 1 in which said means of periodically varying the diameter of said beam includes a pulse source connected to an electrode of said electron gun to vary the diameter of said flooding beam and thereby selectively flood areas of said storage electrode.

3. The storage system set forth in claim 1 in which said storage electrode is an apertured member having a backing electrode and a charge storage surface, a first voltage source connected to the backing electrode of said storage target to provide a pulse of a first time period and in which means for periodically varying to the diameter of said flooding electron beam comprises a second voltage source connected to an electrode of said flooding electron gun to provide a second pulse of a time period shorter than said first time period to selectively flood diiferent areas of said storage electrode during the interval of said first time period.

4. The method of erasing a storage image from a storage grid in a storage display tube comprising the steps of applying an erase pulse to the backing electrode of said storage grid while simultaneously applying a variable voltage waveform to an electrode of a flooding electron beam directed onto said storage grid to vary the diameter of said beam during erase to provide uniformity of erasure over the surface of said storage grid.

References Cited UNITED STATES PATENTS 2,784,342 3/1957 Overbeek 31530 X 2,931,937 4/ 1960 Dufour 3 l512 2,967,969 1/ 1961 Stocker 315l2 RODNEY D. BENNETT, JR., Primary Examiner HERBERT C. WAMSLEY, Assistant Examiner US. Cl. X.R. 

